1. Field of Invention
This invention relates to coating compositions. More particularly, this invention relates to melamine formaldehyde cured, coating compositions which give off reduced levels of formaldehyde.
2. Prior Art
In recent years increasing concerns have been expressed about coating compositions which give off formaldehyde during the curing process. Therefore, there has been an emphasis on developing low formaldehyde or formaldehyde-free coatings.
U.S. Pat. Nos. 4,663,410 and 4,674,611, as well as European Patent 201,693 and Chapter 31 of the American Chemical Society, ACS Symposium Series 367, Cross-Linked Polymers, Chemistry, Properties & Applications, disclose the reaction of a polyacetal with an acrylic resin to produce an acetalated acrylic resin which then may be cross-linked by using a polyhydroxy containing monomer.
A second approach disclosed in U.S. Pat. No. 4,655,841 involves the surface curing of a sizing composition by spraying a diacetal onto a polyhydroxylated resin which is based upon polyvinyl alcohol and polyvinyl acetate.
Yet another approach disclosed in the article includes mixing an hydroxyl polymer with a polyacetal and then self-polymerizing the resulting product in situ.
An inherent disadvantage of many of the prior art polyacetal polymers is that they are color formers unless used in the manner described herein. Accordingly, when acetal/OH reactions are carried out using products which do not conform to the chemical structures described herein, undesirable color formation is possible.
Therefore, it is an object of this invention to prepare compositions which give off reduced amounts of formaldehyde when they cure.
It is another object of this invention to prepare coating compositions based upon hydroxy polymers and monomeric polyacetals.
It is a further object of this invention to prepare coatings based upon polyacetal cross-linking agents which do not form undesirably discolored coatings.
These and other objectives are obtained by carrying out the process of the instant invention using the products described herein.